For preventing collisions between ships and for improving marine traffic control by accurate information of movement of the ships, an automatic identification system (AIS) device has been utilized. The AIS transponder has a function to measure a position, a speed over ground (SOG) and a course over ground (COG) of a ship using a global positioning system (GPS) device. The AIS transponder also has a function to receive AIS information of other ships including identification information (maritime mobile service identity (MMSI) code), ship name, position, SOG, COG, destination and the like, which are received from the other ships using VHF band radios.
For an AIS transponder for a small ship, the SOG and COG of the ship are calculated from difference vectors obtained from a history of GPS positioning results (see Japanese Laid-Open Patent Application No. 2006-65392).
In addition, based on the obtained information of the user ship (ship that is used as a point of origin for calculation) and another ship (target ship), CPA, which is the distance at which the target ship approaches the closest to the user ship, and TCPA, which is the time for the target ship to approach the closest to the user ship, are calculated. When CPA and TCPA are below predetermined values, an alarm is generated to warn the user.
Positioning by GPS includes positioning errors of a few meters to more than ten meters. When the ship is moving faster than a certain speed, effects of the positioning errors are small because a long difference vector is obtained. However, when the ship is stopping or moving at low speed, effects of the positioning errors become significant because the difference vector obtained is short. Therefore, the SOG and COG of the ship vary randomly. For example, when the ship is stopping, the COG varies randomly between 0° and 359.9° in all directions, and the SOG varies randomly between 0 and 3 kt depending on locations of GPS satellites or depending on selections of the GPS satellites that a GPS engine uses for calculation.
As a result, close approaches or collisions cannot be always accurately informed to the user due to the low reliability of the CPA and TCPA when the ship is stopping or moving at low speed. In addition, the CPA and TCPA may randomly vary across a threshold value for the alarm up and down. In such a case, an alarm may be repeatedly executed and stopped and an alarm that was previously executed and stopped may be executed again.
An object of the present invention is to provide a target identification device and a target movement prediction method in which the CPA and TCPA do not randomly vary when a moving object, such as a ship, is stopping or moving at low speed.